Read a binary file using unpack in Python compared with a IDL method

Question

I have a IDL procedure reading a binary file and I try to translate it into a Python routine. The IDL code look like :

a = uint(0)
b = float(0)
c = float(0)
d = float(0)
e = float(0)
x=dblarr(nptx)
y=dblarr(npty)
z=dblarr(nptz)
openr,11,name_file_data,/f77_unformatted
readu,11,a
readu,11,b,c,d,e
readu,11,x
readu,11,y
readu,11,z

it works perfectly. So I'm writing the same thing in python but I can't find the same results (even the value of 'a' is different). Here is my code :

x=np.zeros(nptx,float)
y=np.zeros(npty,float)
z=np.zeros(nptz,float)
with open(name_file_data, "rb") as fb:
    a, = struct.unpack("I", fb.read(4))
    b,c,d,e = struct.unpack("ffff", fb.read(16))
    x[:] = struct.unpack(str(nptx)+"d", fb.read(nptx*8))[:]
    y[:] = struct.unpack(str(npty)+"d", fb.read(npty*8))[:]
    z[:] = struct.unpack(str(nptz)+"d", fb.read(nptz*8))[:]

Hope it will help anyone to answer me.

Update : As suggested in the answers, I'm now trying the module "FortranFile", but I'm not sure I understood everything about its use.

from scipy.io import FortranFile
f=FortranFile(name_file_data, 'r')
a=f.read_record('H')
b=f.read_record('f','f','f','f')

However, instead of having an integer for 'a', I got : array([0, 0], dtype=uint16).

And I had this following error for 'b': Size obtained (1107201884) is not a multiple of the dtypes given (16)

Answer 1

According to a table of IDL data types, UINT(0) creates a 16 bit integer (i.e. two bytes). In the Python struct module, the I format character denotes a 4 byte integer, and H denotes an unsigned 16 bit integer.

Try changing the line that unpacks a to

    a, = struct.unpack("H", fb.read(2))

Unfortunately, this probably won't fix the problem. You use the option /f77_unformatted with openr, which means the file contains more than just the raw bytes of the variables. (See the documentation of the OPENR command for more information about /f77_unformatted.)

You could try to use scipy.io.FortranFile to read the file, but there are no gaurantees that it will work. The binary layout of an unformatted Fortran file is compiler dependent.